The blades for a wind energy plant are for the most part shaped as aerodynamical profiles in order to exploit the wind power optimally. The blades are connected to the hub often via one or more bearings, whereby the loads are transferred to the hub and to a certain degree to the rotor shaft. The loads comprise both the (intentional) rotational forces, centrifugal forces, gravity forces and forces, loads and moments arising out of the action of the wind on the rotor blades. The wind turbine blades can be rigidly connected to the hub, but most modern wind turbines can regulate and adjust the position of the blades for instance by pitch regulation and/or by coning. Therefore, the blades are often connected to the hub via bearings of some kind, for instance a circular pitch bearing allowing each blade to be turned around its pitch axis.
Today, wind turbine blades are typically made of fiber reinforced laminates in order to obtain strong yet light constructions. Such blades are connected to the circular bearing in the hub by means of a number of rods or bolts extending from the blade root in parallel to the pitch axis and being fastened to the blade by bushings or wedges embedded in the blade laminate. In attempts to solve the problem of ensuring a proper engagement of the bushings and obtaining a proper load transfer to the surrounding laminate, different designs of the bushings and the laminate built-up have been suggested, e.g. WO 2003/082551, WO 2004/110862, WO 2006/070171; however, this still remains a field for ongoing research.
The fastening of the rods to the blade and the load transfer from the blade to the hub further requires that the blade be shaped with a right root cylinder of a certain minimum length in which the bushings are placed. The root cylinder is then morphed into the aerodynamically optimized part of the blade. The root section—defined as the blade part extending from the hub and the very root of the blade to the position of the blade profile with maximal chord—can be in the order of 10 m on a 60 m long blade. The disadvantage of the root section is of course that the wind forces are not optimally exploited in this region closest to the hub. In Wobben, WO 2004/097215, this problem is suggested solved by applying a foldable rear edge section or a cladding which is fastened onto the load bearing ‘skeleton’ of the blade.